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South Africa 235 (de Kock 80, Vishwa 4-62, Rajitha 3-68) and 126 for 4 (Elgar 35, Embuldeniya 2-36) lead Sri Lanka 191 (Kusal Perera 51, Steyn 4-48) by 170 runs
Durban: Sri Lanka were bowled out for 191 and were unable to build on a good first day when their inexperienced bowlers surprised the South African batsmen, who could manage only 235 in their first innings.
Steyn and Vernon Philander soon put their stamp on the game when Sri Lanka resumed at 49 for one on Thursday, taking three wickets in the first ten overs of the morning.
Kusal Perera played some bold and occasionally lucky strokes in making 51 off 63 balls for Sri Lanka and the hosts were frustrated when a barrage of short-pitched bowling could not break a stubborn ninth wicket partnership of 32, the joint second-highest of the innings, between debutant Lasith Embuldeniya and Kasun Rajitha.
The stand was eventually ended by a smart run-out of Rajitha by Aiden Markram at short leg.
Steyn took his career tally to 437 Test wickets, passing India's Kapil Dev (434) and drawing level with Stuart Broad of England in joint seventh place on the all-time list.
With bright sunshine following the morning's overcast conditions, the stage seemed set for South Africa to move into a commanding position as Markram struck the ball crisply in making 28 off 31 balls in an opening partnership of 36 with Elgar.
But Markram played one attacking shot too many when an attempted back foot force off Rajitha flew to Kusal Mendis at second slip.
Three more wickets fell reasonably cheaply as South Africa's top-order batting again proved fragile.
Hashim Amla edged Vishwa Fernando to gully before Embuldeniya followed up his courageous innings of 24 off 63 balls by dismissing Temba Bavuma grabs advantage for south Africa and Elgar with his left-arm spin bowling.
Bavuma was leg before wicket for three when he missed a sweep and Elgar fell for 35 to a superb return catch when he skipped down the wicket and hit the ball hard but close enough for the bowler to take a reflex catch to his left.
